well, the following procedure is not usual way, but sharing for your information -
You can install a Local Relay Module and connect PLC output to this Relay Module. Connect 240V Ac power supply to the relay and power the Valves with this intermediate relay module.
To work this out, first check whether PLC output power able to switch ON and switch OFF the relay module.
In First Method we are using PLC power to energize and de-energize the relay module. as you have more induction voltage in cable, so first check whether you are able to make relay ON & OFF from PLC.
SECOND METHOD -
Instead of sending 240V AC power from the PLC, change the loop to DRY i.e. only continuity, when PLC sends signal say the output contact is NC contact and no signal then NO contact in the PLC
Now install Intermediate Relay Module at the Valve (field). Use the NO & NC signals from PLC to control this Relay module with auxiliary/external control power supply.
Now use 240V AC power supply and wire the Valves via this Intermediate Relay Module at the Field.
In Second method, we are using PLC output either NO or NC only. There will be NO 240V Ac output from PLC, you have to make wiring change in PLC.
Note : You can use single 240V AC power supply for Valve’s powering and for relay control supply. OR. use different power supplies, as per your availability. In my view, different power supplies will be good.
If possible you can change the Junction box where some spares available in main cable. you have to lay branch cables from valve to that junction box.
YOU CAN CONNECT A SNUBBER CIRCUIT OR RC-CIRCUIT ACROSS RELAY TERMINALS TO REDUCE THE INDUCTION VOLTAGE IMPACT FOR ANY OF THE METHOD